The present invention relates in general to die casting machines. More specifically, the present invention provides an apparatus and a method for controlling the speed of a plunger slideably mounted inside of a sleeve provided as part of the machine, thereby controlling the flow rate and pressure of melted metal being charged into a casting cavity by the plunger.
In general, the flow rate and pressure of melted metal being charged into the cavity of a die casting machine influence the quality of molded articles formed therein. A conventional injection process is essentially a two step process. In the first step, the plunger is controlled so as to move forward at a first (low) speed thereby preventing melted metal from being rough in the sleeve as the plunger moves, after the metal is poured in the sleeve.
At the second (last) step, the plunger is controlled so as to move forward at a second (high) speed for molding the melted metal into the cavity formed by male and female dies at a high rate of flow.
Referring to FIGS. 1(A), (B) and (C) there is illustrated the injection process mentioned above. Numeral 10 designates a hydraulic cylinder for carrying out the injection process. A piston 12 moves within cylinder 10 with a plunger 14 rigidly connected to the piston. A plunger tip 16 is mounted inside of a sleeve 18 into which a melted metal 22, such as, for example, melted aluminum is poured through an opening 19 before the plunger tip is moved to force the melted metal into the molding cavity (to the left as shown in FIG. 1(A)). Melted metal 22 is charged through a gate 24 into a cavity 21 formed by dies 20 and 23. A vent 26 provides an escape for air in cavity 21 as metal fills the cavity.
FIG. 1(A) shows the state of the die casting machine just after pouring melted metal 22 into sleeve 18. Plunger tip 16 is beginning to move at a low speed for carrying out the first step of the molding process.
When plunger tip 16 moves, passing through opening 19, to a position at which sleeve 18 is filled with melted metal 22 and the level of metal reaches up to gate 24 as shown in FIG. 1(B), the plunger tip is controlled to increase its speed to a high speed. Melted metal in the sleeve is molded into cavity 21 with a high rate of flow as shown in FIG. 1(C).
Since melted metal 22 becomes solidified in a very short time after it enters into cavity 21, it is necessary for the melted metal to be molded into cavity 21 with a considerably high rate near gate 24. At such a high rate of metal flow, air in cavity 21 cannot completely escape through vent 26, and melted metal 22 is charged into cavity 21 from a portion of gate 24 as a jet. This results in pin holes being formed in articles molded in cavity 21. These pin holes are formed by remaining air or a blow hole is formed by involving the air therein.